The normal spine when viewed in an anterior posterior direction is straight. The same spine, when viewed from the lateral view has a series of curvatures. The thoracic curvature has a convexity posteriorly (in the sagittal plane). The lumbar spine has a curvature with the convexity directed anteriorly. Scoliosis, traditionally, has been discussed as a side bending (in the coronal plane) of the normally straight spine when viewed from the anterior-posterior direction. Depending on the etiology, there may be just one primary curve having smaller secondary compensatory curves, or there may be several primary curves. The stiffness of the curve varies between individuals dependent upon the amount of bone deformity and ligamentous contracture that has occurred over a period of time. Harrington's first apparatus for aligning the spine was a straight rod attached at a single site top and bottom. This did an excellent job of straightening the lateral curvature (coronal plane) but ignored the normal sagittal plane curvatures of the spine. This resulted in a worsening of the spinal alignment in the sagittal plane in many cases.
Scoliosis now is fully appreciated to be a deformity of the spine in three planes. This deformity results in the appearance of (1) a lateral bend on the anterior posterior view, (2) a loss of the normal convexity posterior curvature of the thoracic area or a straightening of the spine on the lateral view and (3) a rotational deformity of the apical vertebra, wherein the transverse process of the apical vertebra on the convex side rotates posteriorly and the transverse process of the apical vertebra on the concave side rotates anteriorly.
Cotrel attempted to deal with all three aspects of this deformity by changing the instrumentation with the Cotrel-Dubousset (C-D) technique. The C-D instrumentation for the correction and stabilization of spinal deformity was introduced to the Scoliosis Research Society in 1984. The C-D instrumentation was subsequently modified by Texas Scottish Rite Hospital (TSRH) in 1985. Both the C-D and TSRH instrumentation include a curved rod designed with multiple hooks for attachment to the spine. The rod is first applied to the concave side of the curve. The instrumentation allows rotation between the hooks and the rod so that the rod is placed in the spine in a scoliotic position, that is with the curvature of the rod matching the curvature of the deformity. The rod is then rotated 90 degrees converting the scoliosis into kyphosis. This helps decrease the lateral curvature of the spine in the anterior posterior direction and also helps increase the insufficient curvature in the sagittal plane. It initially was hoped that this also would reduce the rotation of the apical vertebra. The results in this last case have been disappointing. Since this technique still relies primarily on a single rod pulling in a single direction for correction, it is understandable how this cannot effectively derotate the vertebra.
In view of the shortcomings of the existing procedures for treating scoliotic spines, there is a need for improvements which will allow practitioners to apply forces lifting the concave side while simultaneously compressing the convex side, thus imparting a derotational torque while at the same time correcting both saggital and coronal plane deformity.